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Integration of renewable energy sources such as solar photovoltaic (PV) generation with variable power demand systems like residential electricity consumption requires the use of a high efficiency electrical energy system such as a battery. In the present study, such integration has been studied using vanadium redox flow battery (VRFB) as the energy storage system with specific focus on the sizing of the power and energy storage capacities of the system components. Experiments have been carried out with a seven-day simulated solar insolation and residential load characteristics using a 1 kW VRFB stack and variable amounts of electrolyte volume. Several scenarios have been simulated using power and energy scale factors. Battery response, in terms of its power, state of charge and efficiency, has been monitored in each run. Results show that the stack power rating should be based on peak charging characteristics while the volume of electrolyte should be based on the expected daily energy discharge through the battery. The PV source itself should be sized at about 25% more energy rating than the average daily load. The ability to design a VRFB with a high power-to-energy ratio makes it particularly attractive for PV-load integration.

Progress in renewable energy production has directed interest in advanced developments of energy storage systems. The all-vanadium redox flow battery (VRFB) is one of the attractive technologies for large scale energy storage due to its design versatility and scalability, longevity, good round-trip efficiencies, stable capacity and safety. Despite these advantages, the deployment of the vanadium battery has been limited due to vanadium and cell material costs, as well as supply issues. Improving stack power density can lower the cost per kW power output and therefore, intensive research and development is currently ongoing to improve cell performance by increasing electrode activity, reducing cell resistance, improving membrane selectivity and ionic conductivity, etc. In order to evaluate the cell performance arising from this intensive R&D, numerous physical, electrochemical and chemical techniques are employed, which are mostly carried out ex situ, particularly on cell characterizations. However, this approach is unable to provide in-depth insights into the changes within the cell during operation. Therefore, in situ diagnostic tools have been developed to acquire information relating to the design, operating parameters and cell materials during VRFB operation. This paper reviews in situ diagnostic tools used to realize an in-depth insight into the VRFBs. A systematic review of the previous research in the field is presented with the advantages and limitations of each technique being discussed, along with the recommendations to guide researchers to identify the most appropriate technique for specific investigations.

A low-pressure drop stack design with minimal shunt losses was explored for vanadium redox flow batteries, which, due to their low energy density, are used invariably in stationary applications. Three kilowatt-scale stacks, having cell sizes in the range of 400 to 1500 cm2, were built with thick graphite plates grooved with serpentine flow fields and external split manifolds for electrolyte circulation, and they were tested over a range of current densities and flow rates. The results show that stacks of different cell sizes have different optimal flow rate conditions, but under their individual optimal flow conditions, all three cell sizes exhibit similar electrochemical performance including stack resistivity. Stacks having larger cell sizes can be operated at lower stoichiometric factors, resulting in lower parasitic pumping losses. Further, these can be operated at a fixed flow rate for power variations of ±25% without any significant changes in discharge capacity and efficiency, this is attributed to the use of serpentine flow fields, which ensure uniform distribution of the electrolyte over a range of flow rates and cell sizes.

Accurately predicting battery behavior, while using low input data, is highly desirable in embedded simulation architectures like grid or integrated energy system analysis. Currently, the available vanadium redox flow battery (VRFB) models achieve highly accurate predictions of electrochemical behavior or control algorithms, while the optimization of the required input data scope is neglected. In this study, a parametrization tool for a DC grey box simulation model is developed using measurements with a 10 kW/100 kWh VRFB. An objective function is applied to optimize the required input data scope while analyzing simulation accuracy. The model is based on a differential-algebraic system, and an optimization process allows model parameter estimation and verification while reducing the input data scope. Current losses, theoretical storage capacity, open circuit voltage, and ohmic cell resistance are used as fitting parameters. Internal electrochemical phenomena are represented by a self-discharge current while material related losses are represented by a changing ohmic resistance. Upon reducing input data the deviation between the model and measurements shows an insignificant increase of 2% even for a 60% input data reduction. The developed grey box model is easily adaptable to other VRFB and is highly integrable into an existing energy architecture.

The VO2+/VO2+ redox couple commonly employed on the positive terminal of the all-vanadium redox flow battery was investigated at various states of charge (SOC) and H2SO4 supporting electrolyte concentrations. Electron paramagnetic resonance was used to investigate the VO2+ concentration and translational and rotational diffusion coefficient (DT, DR) in both bulk solution and Nafion membranes. Values of DT and DR were relatively unaffected by SOC and on the order of 10&minus, 10 m2s&minus, 1. Cyclic voltammetry measurements revealed that no significant changes to the redox mechanism were observed as the state of charge increased, however, the mechanism does appear to be affected by H2SO4 concentration. Electron transfer rate (k0) increased by an order of magnitude (10&minus, 6 ms&minus, 1 to 10&minus, 8 ms&minus, 1) for each H2SO4 concentrations investigated (1, 3 and 5 M). Analysis of cyclic voltammetry switching currents suggests that the technique might be suitable for fast determination of state of charge if the system is well calibrated. Membrane uptake and permeability measurements show that vanadium absorption and crossover is more dependent on both acid and vanadium concentration than state of charge. Vanadium diffusion in the membrane is about an order of magnitude slower (~10&minus, 11 m2s&minus, 1) than in solution (~10&minus, 1).

Common issues aqueous-based vanadium redox flow batteries (VRFBs) face include low cell voltage due to water electrolysis side reactions and highly corrosive and environmentally unfriendly electrolytes (3 to 5 M sulfuric acid). Therefore, this investigation looks into the comparison of a highly conductive ionic liquid with a well-studied deep eutectic solvent (DES) as electrolytes for non-aqueous VRFBs. The latter solvent gives 50% higher efficiency and capacity utilization than the former. These figures of merit increase by 10% when nitrogen-doped graphene (N-G)-modified carbon papers, via a one-step binder-free electrophoretic deposition process, are used as electrodes. X-ray computed tomography confirms the enhancement of electrochemical surface area of the carbon electrodes due to N-G while electrochemical impedance spectra show the effect of its higher conductivity on improving RFB performance. Finally, potential strategies for the scaling-up of DES-based VRFBs using a simple economical model are also briefly discussed. From this study, it is deduced that more investigations on applying DESs as non-aqueous electrolytes to replace the commonly used acetonitrile may be a positive step forward because DESs are not only cheaper but also safer to handle, far less toxic, non-flammable, and less volatile than acetonitrile.

We describe a ZnBr2 single cell which has a highly modular symmetrical structure. With designed polyethylene shell frames, membrane frame and composite titanium-carbon felt electrodes, it has a higher energy density and is more flexible compared with traditional flow batteries. We repeatedly tested its performance, which showed good tightness, high reliability and a high energy efficiency of 75%. Due to the special symmetrical structure and modular design, it is easy to assemble and disassemble, which makes it suitable as a test platform for electrodes, membranes and electrolyte performance testing. The designed modular flow cell has low cost and high energy density, and can provide good guidance for flow battery research.

A vanadium oxygen fuel cell is a modified form of a conventional vanadium redox flow battery (VRFB) where the positive electrolyte (VO2+/VO2+ couple) is replaced by the oxygen reduction (ORR) process. This potentially allows for a significant improvement in energy density and has the added benefit of overcoming the solubility limits of V (V) at elevated temperatures, while also allowing the vanadium negative electrolyte concentration to increase above 3 M. In this paper, a vanadium oxygen fuel cell with vanadium electrolytes with a concentration of up to 3.6 M is reported with preliminary results presented for different electrodes over a range of current densities. Using precipitation inhibitors, the concentration of vanadium can be increased considerably above the commonly used 2 M limit, leading to improved energy density.

In this study, 1.6 M vanadium electrolytes in the oxidation forms V(III) and V(V) were prepared from V(IV) in sulfuric (4.7 M total sulphate), V(IV) in hydrochloric (6.1 M total chloride) acids, as well as from 1:1 mol mixture of V(III) and V(IV) (denoted as V3.5+) in hydrochloric (7.6 M total chloride) acid. These electrolyte solutions were investigated in terms of performance in vanadium redox flow battery (VRFB). The half-wave potentials of the V(III)/V(II) and V(V)/V(IV) couples, determined by cyclic voltammetry, and the electronic spectra of V(III) and V(IV) electrolyte samples, are discussed to reveal the effect of electrolyte matrix on charge-discharge behavior of a 40 cm2 cell operated with 1.6 M V3.5+ electrolytes in sulfuric and hydrochloric acids. Provided that the total vanadium concentration and the conductivity of electrolytes are comparable for both acids, respective energy efficiencies of 77% and 72–75% were attained at a current density of 50 mA∙cm−2. All electrolytes in the oxidation state V(V) were examined for chemical stability at room temperature and +45 °C by titrimetric determination of the molar ratio V(V):V(IV) and total vanadium concentration.

Segmented cells enable real time visualization of the flow distribution in vanadium redox flow batteries by local current or voltage mapping. The lateral flow of current within thick porous electrodes, however, impairs the local resolution of the detected signals. In this study, the open circuit voltage immediately after the cessation of charge/discharge is used for the mapping of reactant conversion. This quantity is not hampered by lateral flow of current and can be conveniently transformed to the corresponding state of charge. The difference between theoretically calculated and experimentally determined conversion (change in the state of charge) across the electrode is used to determine local variations in conversion efficiency. The method is validated by systematic experiments using electrodes with different modifications, varying current densities and flow configurations. The procedure and the interpretation are simple and scalable to any size of flow cell. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Published version

In this study, we have proposed a novel concept of hybrid flow batteries consisting of a molten Na-Cs anode and an aqueous NaI catholyte separated by a NaSICON membrane. A number of carbonaceous electrodes are studied using cyclic voltammetry (CV) for their potentials as the positive electrode of the aqueous NaI catholyte. The charge transfer impedance, interfacial impedance and NaSICON membrane impedance of the Na-Cs ‖ NaI hybrid flow battery are analyzed using electrochemical impedance spectroscopy. The performance of the Na-Cs ‖ NaI hybrid flow battery is evaluated through galvanostatic charge/discharge cycles. This study demonstrates, for the first time, the feasibility of the Na-Cs ‖ NaI hybrid flow battery and shows that the Na-Cs ‖ NaI hybrid flow battery has the potential to achieve the following properties simultaneously: (i) An aqueous NaI catholyte with good cycle stability, (ii) a durable and low impedance NaSICON membrane for a large number of cycles, (iii) stable interfaces at both anode/membrane and cathode/membrane interfaces, (iv) a molten Na-Cs anode capable of repeated Na plating and stripping, and (v) a flow battery with high Coulombic efficiency, high voltaic efficiency, and high energy efficiency.

Vanadium redox flow batteries (VRFBs) offer great promise as a safe, cost effective means of storing electrical energy on a large scale and will certainly have a part to play in the global transition to renewable energy. To unlock the full potential of VRFB systems, however, it is necessary to improve their power density. Unconventional stack design shows encouraging possibilities as a means to that end. Presented here is the novel concept of variable porous electrode compression, which simulations have shown to deliver a one third increase in minimum limiting current density together with a lower pressure drop when compared to standard uniform compression cell designs.

The vanadium redox flow battery (VRFB) is one of the most mature and commercially available electrochemical technologies for large-scale energy storage applications. The VRFB has unique advantages, such as separation of power and energy capacity, long lifetime (>20 years), stable performance under deep discharge cycling, few safety issues and easy recyclability. Despite these benefits, practical VRFB operation suffers from electrolyte imbalance, which is primarily due to the transfer of water and vanadium ions through the ion-exchange membranes. This can cause a cumulative capacity loss if the electrolytes are not rebalanced. In commercial systems, periodic complete or partial remixing of electrolyte is performed using a by-pass line. However, frequent mixing impacts the usable energy and requires extra hardware. To address this problem, research has focused on developing new membranes with higher selectivity and minimal crossover. In contrast, this study presents two alternative concepts to minimize capacity fade that would be of great practical benefit and are easy to implement: (1) introducing a hydraulic shunt between the electrolyte tanks and (2) having stacks containing both anion and cation exchange membranes. It will be shown that the hydraulic shunt is effective in passively resolving the continuous capacity loss without detrimentally influencing the energy efficiency. Similarly, the combination of anion and cation exchange membranes reduced the net electrolyte flux, reducing capacity loss. Both approaches work efficiently and passively to reduce capacity fade during operation of a flow battery system.

The number of Stationary Battery Systems (SBS) connected to various power distribution networks across the world has increased drastically. The increase in the integration of renewable energy sources is one of the major contributors to the increase in the number of SBS. SBS are also used in other applications such as peak load management, load-shifting, voltage regulation and power quality improvement. Accurately modeling the charging/discharging characteristics of such SBS at various instances (charging/discharging profile) is vital for many applications. Capacity loss due to the aging of the batteries is an important factor to be considered for estimating the charging/discharging profile of SBS more accurately. Empirical modeling is a common approach used in the literature for estimating capacity loss, which is further used for estimating the charging/discharging profiles of SBS. However, in the case of SBS used for renewable integration and other grid related applications, machine-learning (ML) based models provide extreme flexibility and require minimal resources for implementation. The models can even leverage existing smart meter data to estimate the charging/discharging profile of SBS. In this paper, an analysis on the performance of different ML approaches that can be applied for lithium iron phosphate battery systems and vanadium redox flow battery systems used as SBS is presented for the scenarios where the aging of individual cells is non-uniform. Published version

One of the most important parameters for the design of redox flow batteries is a uniform distribution of the electrolyte solution over the complete electrode area. The performance of redox flow batteries is usually investigated by general measurements of the cell in systematic experimental studies such as galvanostatic charge-discharge cycling. Local inhomogeneity within the electrode cannot be locally-resolved. In this study a printed circuit board (PCB) with a segmented current collector was integrated into a 40 cm2 all-vanadium redox flow battery to analyze the locally-resolved current density distribution of the graphite felt electrode. Current density distribution during charging and discharging of the redox flow battery indicated different limiting influences. The local current density in redox flow batteries mainly depends on the transport of the electrolyte solution. Due to this correlation, the electrolyte flow in the porous electrode can be visualized. A PCB electrode can easily be integrated into the flow battery and can be scaled to nearly any size of the electrode area. The carbon coating of the PCB enables direct contact to the corrosive electrolyte, whereby the sensitivity of the measurement method is increased compared to state-of-the-art methods.

The battery energy storage system has become an indispensable part of the current electricity network due to the vast integration of renewable energy sources (RESs). This paper proposes an optimal charging method of a vanadium redox flow battery (VRB)-based energy storage system, which ensures the maximum harvesting of the free energy from RESs by maintaining safe operations of the battery. The VRB has a deep discharging capability, long cycle life, and high energy efficiency with no issues of cell-balancing, which make it suitable for large-scale energy storage systems. The proposed approach determines the appropriate charging current and the optimal electrolyte flow rate based on the available time-varying input power. Moreover, the charging current is bounded by the limiting current, which prevents the gassing side-reactions and protects the VRB from overcharging. The proposed optimal charging method is investigated by simulation studies using MATLAB/Simulink.

This article reports extensive studies of a Vycor® porous glass (VPG) membrane as an ion separator for an all-vanadium redox flow battery (VRFB). The VPG membrane had an average pore size of 4 nm and porosity of ~28%. The VPG ion separator exhibited higher proton diffusivity but lower conductivity than the Nafion® 117 membrane because the former is intrinsically nonionic. The VRFB equipped with the VPG ion separator (VPG-VRFB) exhibited much better stability during long-term cyclic operation than the VRFB equipped with the Nafion-117 membrane (Nafion-VRFB) because the ionic Nafion membrane could be contaminated by vanadium ions exchanged into the water channels. This increases its area specific resistance, while the VPG does not have ion exchange capacity and hence has less vanadium ion contamination. The VPG-VRFB was found to outperform the Nafion-VRFB in energy efficiency (EE) during long-term cyclic operation although the former underperformed the latter in the initial period of continued operation. The VPG ion separator also showed markedly better thermal stability and temperature tolerance than the Nafion membrane as indicated by the significantly smaller losses of EE and discharge capacity for the VPG-VRFB than for the Nafion-VRFB after operating at 45 °C. The outstanding temperature tolerance of the VPG ion separator is due primarily to its rigid and non-swelling pore structure and nonionic nature, which are highly resistant to thermal distortion and vanadium ion contamination. The excellent temperature tolerance of the VPG may be useful for applications where temperature control is difficult.

Vanadium redox-flow batteries are a promising energy storage technology due to their safety, long-term stability, and independent adjustability of power and capacity. However, the vanadium crossover through the membrane causes a self-discharge, which results in a capacity shift towards one half cell. This leads to a gradual decrease in its efficiency over time. Capacity balancing methods for compensation of this effect require a reliable online state of charge (SoC) monitoring. Most common methods cannot provide exact values of the individual concentration of each species in both electrolytes. In particular, the state of the positive electrolyte cannot yet be precisely determined. In this work, an amperometric SoC monitoring is proposed as a new approach. First, the suitability of the principle is investigated with a rotating disc electrode (RDE). Then, a sensor based on a gas diffusion layer (GDL) is developed and tested in the positive electrolyte. The dependencies between oxidative current and V(IV)-concentration are examined as well as those between reduction current and V(V)-concentration. Using both relationships, a reliable measurement of all relevant concentrations is possible.

Hydrogen-bromine (H2-Br2) redox flow batteries (RFBs) have gained a lot of interest due to their advantages in mitigating the performance shortcomings of conventional zinc-bromine and vanadium flow batteries. Various carbon materials have been tested in H2-Br2 RFBs as bromine electrodes. However, a comparative study among the different carbon materials has not been reported in the literature. This work reports, for the first time, an evaluation of carbon papers, felt and cloth in a three-electrode half-cell setup as potential bromine electrodes, in pristine and thermally treated state. A systematic evaluation was performed by comparing the surface morphologies, kinetic parameters, polarisation curves and stability tests of different carbon electrodes. Thermally treated graphite felt electrode demonstrated the best electrochemical performance as bromine electrode owing to its improved surface area, hydrophilicity and intrinsic activity. Further in-depth studies will shed important insights, which will help understand the electrode characteristics for future bromine battery design. The current study will assist in evaluating the performance of upcoming novel electrode materials in a three-electrode assembly.